run-tests: ignore failures from rmtree

rmtree() may fail under certain conditions. We ignore failures at the
individual test level because they can interfere with test execution.
Furthermore, we'll reattempt deletion in the high-level test runner
after all tests have finished.

Note that this patch is not a code refactor like most of the patches
before it. This change logically makes sense given the execution
behavior of the tests.

tests/run-tests.py

@@ -383,6 +383,8 b' class Test(object):'
             killdaemons(entry)
 
         if self._threadtmp and not self._options.keep_tmpdir:
+            # Ignore failures here. The rmtree() in the higher level runner
+            # will try again.
             shutil.rmtree(self._threadtmp, True)
 
     def setUp(self):
@@ -525,7 +527,7 b' class Test(object):'
         self._daemonpids = []
 
         if not self._options.keep_tmpdir:
-            shutil.rmtree(self._testtmp)
+            shutil.rmtree(self._testtmp, True)
 
         if (self._ret != 0 or self._out != self._refout) and not self._skipped \
             and not self._options.debug and self._out: